Field of the Invention
The present invention relates to an aluminum electrolytic capacitor for use in various electronic devices.
A configuration of the aluminum electrolytic capacitor according to the present invention is shown in FIG. 1. The FIGURE is a partial cross-sectional perspective view.
A capacitor element 19 in FIG. 1 is prepared by winding an anode foil 11 and a cathode foil 12 via a separator 13, wherein the anode foil 11 is obtained by etching a surface of an aluminum foil to increase its effective surface area and subjecting the surface to a chemical conversion treatment to form a dielectric oxide film thereon, and the cathode foil 12 is obtained by etching a surface of an aluminum foil. The aluminum electrolytic capacitor 19 can be produced by connecting an anode lead terminal 15 and an cathode lead terminal 16 respectively to the anode foil 11 and the cathode foil 12, impregnating the capacitor element with a driving electrolyte solution 14, placing the capacitor element 19 in a metal case 18 such as aluminium case, and sealing the opening with a sealing plate 17, for example, of rubber.
Many ion-conductive liquids, including those containing an organic solvent such as ethylene glycol or γ-butyllactone and an electrolyte such as boric acid or ammonium borate, have been used as the driving electrolyte solution 14. Alternatively, non-aqueous electrolyte solutions containing one of dibasic acids having a side-chain such as azelaic acid, butyloctanedicarboxylic acid, and 5,6-decanedicarboxylic acid, or the salts thereof as the electrolyte, which allows reduction of water in the driving electrolyte solution 14, are said to be effective in suppressing unintended release of the valves in the aluminum electrolytic capacitor due to the increase in internal pressure by water even in an environment of 100° C. or more.
However, the nonaqueous driving electrolyte solutions 14 causes faster degradation of the electrolyte solution in an esterification reaction at high temperature, and thus dibasic acids having a side-chains such as butyloctanedicarboxylic acid, 5,6-decanedicarboxylic acid, and 1,7-octanedicarboxylic acid or the salts thereof have been used in an electrolyte effective in suppressing the esterification reaction.
Prior art references relevant to the present invention include, for example, Jpn. Unexamined Patent Publication Nos. 05-226189 and 2001-313234.
Recently, for aluminum electrolytic capacitors for use in anti-harmonic distortion circuits and vehicles, there exists a need for a driving electrolyte solution 14 that is higher in electric conductivity, superior in the spark voltage and the life at high temperature, resistant to the breakdown of dielectric oxide films on electrodes, superior in film-repairing ability for repairing the defects once formed in the dielectric oxide film (hereinafter, referred to as chemical self-restoring ability), and capable of suppressing chemical reactions at high temperature.
However, when a dibasic acid having a side-chain or the salt thereof is used as the electrolyte of a driving electrolyte solution 14, the capacitor is likely to be less heat resistance at high temperature, because the dibasic acid has a side-chain only in the vicinity of the carboxyl group at one side. Also, the electric conductivity of the electrolyte solution is likely to be lowering, which may cause deteriorating the performance of the aluminum electrolytic capacitor, because the carboxyl group of the electrolyte reacts more with an alcohol such as ethylene glycol in the esterification reaction when the capacitor is used for a prolonged period.
In addition, the compounds above have an advantage that increase in the amount and the molecular weight thereof leads to increase in spark voltage, but on the contrary, become less soluble in organic solvents especially at lower temperature, resulting in precipitation. This restricts the range of the content or the molecular weight of the compound. Accordingly, these compounds have good solubility at lower temperature when they have a molecular weight of 1,000 or less. However, as described above, reduction of the molecular weight lead to an insufficient spark voltage and possible short-circuit explosion during aging of the product. Thus, there exist trade-off problems when these compounds are used for an electrolyte.
Additionally, papers commonly used as separators 13 for general aluminum electrolytic capacitors such as Manila paper, kraft paper, and esparto paper have a high density. Reduction of the density is likely to be less short-circuit resistance and to lower tensile strength.